Smolin contributed to loop quantum gravity (LQG) in collaborative work with Ted Jacobson, Carlo Rovelli, Louis Crane, Abhay Ashtekar and others. LQG is an approach to the unification of quantum mechanics with general relativity which utilizes a reformulation of general relativity in the language of gauge field theories, which allows the use of techniques from particle physics, particularly the expression of fields in terms of the dynamics of loops. (See main page loop quantum gravity.) With Rovelli he discovered the discreteness of areas and volumes and found their natural expression in terms of a discrete description of quantum geometry in terms of spin networks. In recent years he has focused on connecting LQG to phenomenology by developing implications for experimental tests of spacetime symmetries as well as investigating ways elementary particles and their interactions could emerge from spacetime geometry.

Smolin is among those theorists who have proposed that the effects of quantum gravity can be experimentally probed by searching for modifications in special relativity detected in observations of high energy astrophysical phenomena. These include very high energy cosmic rays and photons and neutrinos from gamma ray bursts. Among Smolin’s contributions are the coinvention of doubly special relativity (with João Magueijo, independently of work by Giovanni Amelino-Camelia) and of relative locality (with Amelino-Camelia, Laurent Freidel and Jerzy Kowalski-Glikman).

Smolin's hypothesis of cosmological natural selection, also called the fecund universes theory, suggests that a process analogous to biological natural selection applies at the grandest of scales. Smolin published the idea in 1992 and summarized it in a book aimed at a lay audience called The Life of the Cosmos.

Black holes have a role in natural selection. In fecund theory a collapsing[clarification needed]black hole causes the emergence of a new universe on the "other side", whose fundamental constant parameters (masses of elementary particles, Planck constant, elementary charge, and so forth) may differ slightly from those of the universe where the black hole collapsed. Each universe thus gives rise to as many new universes as it has black holes. The theory contains the evolutionary ideas of "reproduction" and "mutation" of universes, and so is formally analogous to models of population biology.

Alternatively, black holes play a role in cosmological natural selection by reshuffling only some matter affecting the distribution of elementary quark universes. The resulting population of universes can be represented as a distribution of a landscape of parameters where the height of the landscape is proportional to the numbers of black holes that a universe with those parameters will have. Applying reasoning borrowed from the study of fitness landscapes in population biology, one can conclude that the population is dominated by universes whose parameters drive the production of black holes to a local peak in the landscape. This was the first use of the notion of a landscape of parameters in physics.

"I'm not sure why Smolin's idea didn't attract much attention. I actually think it deserved far more than it got."[6]

However, Susskind also argued that, since Smolin's theory relies on information transfer from the parent universe to the baby universe through a black hole, it ultimately makes no sense as a theory of cosmological natural selection.[6] According to Susskind and many other physicists, the last decade of black hole physics has shown us that no information that goes into a black hole can be lost.[6] Indeed, the debate over this issue has been resolved with Stephen Hawking, the largest proponent of the idea that information is lost in a black hole, reversing his position.[6] In this light, information transfer from the parent universe into the baby universe through a black hole is not conceivable.[6]

In a critical review of The Life of the Cosmos, astrophysicist Joe Silk suggested that our universe falls short by about four orders of magnitude from being maximal for the production of black holes.[7] In his book Questions of Truth, particle physicist John Polkinghorne puts forward another difficulty with Smolin's thesis: one cannot impose the consistent multiversal time required to make the evolutionary dynamics work, since short-lived universes with few descendants would then dominate long-lived universes with many descendants.[8] Smolin responded to these criticisms in Life of the Cosmos, and later scientific papers.

When Smolin published the theory in 1992, he proposed as a prediction of his theory that no neutron star should exist with a mass of more than 1.6 times the mass of the sun.[citation needed] Later this figure was raised to two solar masses following more precise modeling of neutron star interiors by nuclear astrophysicists. If a more massive neutron star was ever observed, it would show that our universe's natural laws were not tuned for maximal black hole production, because the mass of the strange quark could be retuned to lower the mass threshold for production of a black hole. A 2-solar-mass pulsar was discovered in 2010.[9]

In 1992 Smolin also predicted that inflation, if true, must only be in its simplest form, governed by a single field and parameter. Both predictions have held up, and they demonstrate Smolin’s main thesis: that the theory of cosmological natural selection is Popper falsifiable.

Smolin has contributed to the philosophy of physics through a series of papers and books that advocate the relational, or Leibnizian, view of space and time. Since 2006, he has collaborated with the Brazilian philosopher and Harvard Law School professor Roberto Mangabeira Unger on the issues of the reality of time and the evolution of laws; in 2014 they published a book, its two parts being written separately.[10]

A book length exposition of Smolin's philosophical views appeared in April 2013. Time Reborn argues that physical science has made time unreal while, as Smolin insists, it is the most fundamental feature of reality: "Space may be an illusion, but time must be real" (p. 179). An adequate description according to him would give a Leibnizian universe: indiscernibles would not be admitted and every difference should correspond to some other difference, as the principle of sufficient reason would have it. A few months later a more concise text has been made available in a paper with the title Temporal Naturalism.[11]

Smolin's 2006 book The Trouble with Physics explored the role of controversy and disagreement in the progress of science. It argued that science progresses fastest if the scientific community encourages the widest possible disagreement among trained and accredited professionals prior to the formation of consensus brought about by experimental confirmation of predictions of falsifiable theories. He proposed that this meant the fostering of diverse competing research programs, and that premature formation of paradigms not forced by experimental facts can slow the progress of science.

As a case study, The Trouble with Physics focused on the issue of the falsifiability of string theory due to the proposals that the anthropic principle be used to explain the properties of our universe in the context of the string landscape. The book was criticized by the physicists Joseph Polchinski[12] and other string theorists.

In his earlier book Three Roads to Quantum Gravity (2002), Smolin stated that loop quantum gravity and string theory were essentially the same concept seen from different perspectives. In that book, he also favored the holographic principle. The Trouble with Physics, on the other hand, was strongly critical of the prominence of string theory in contemporary theoretical physics, which he believes has suppressed research in other promising approaches. Smolin suggests that string theory suffers from serious deficiencies and has an unhealthy near-monopoly in the particle theory community. He called for a diversity of approaches to quantum gravity, and argued that more attention should be paid to loop quantum gravity, an approach Smolin has devised. Finally, The Trouble with Physics is also broadly concerned with the role of controversy and the value of diverse approaches in the ethics and process of science.

In the same year that The Trouble with Physics was published, Peter Woit published a book for nonspecialists whose conclusion was similar to Smolin's, namely that string theory was a fundamentally flawed research program.[13]

The following describes Smolin's personal opinions and by no means consist on scientific established facts, not even at the theoretical level of mathematical consistency and consistency with verified theories. Smolin's view on the nature of time:

"More and more, I have the feeling that quantum theory and general relativity are both deeply wrong about the nature of time. It is not enough to combine them. There is a deeper problem, perhaps going back to the beginning of physics."[14]

"I am convinced that quantum mechanics is not a final theory. I believe this because I have never encountered an interpretation of the present formulation of quantum mechanics that makes sense to me. I have studied most of them in depth and thought hard about them, and in the end I still can't make real sense of quantum theory as it stands."[15]

In a 2009 article, Smolin has articulated the following philosophical views (the sentences in italics are quotations):

There is only one universe. There are no others, nor is there anything isomorphic to it. Smolin denies the existence of a "timeless" multiverse. Neither other universes nor copies of our universe — within or outside — exist.[clarification needed] No copies can exist within the universe, because no subsystem can model precisely the larger system it is a part of. No copies can exist outside the universe, because the universe is by definition all there is. This principle also rules out the notion of a mathematical object isomorphic in every respect to the history of the entire universe, a notion more metaphysical than scientific.

All that is real is real in a moment, which is a succession of moments. Anything that is true is true of the present moment. Not only is time real, but everything that is real is situated in time. Nothing exists timelessly.

Everything that is real in a moment is a process of change leading to the next or future moments. Anything that is true is then a feature of a process in this process causing or implying future moments. This principle incorporates the notion that time is an aspect of causal relations. A reason for asserting it, is that anything that existed for just one moment, without causing or implying some aspect of the world at a future moment, would be gone in the next moment. Things that persist must be thought of as processes leading to newly changed processes. An atom at one moment is a process leading to a different or a changed atom at the next moment.

Smolin views rejecting the idea of a creator as essential to cosmology.[16] He also opposes the anthropic principle, which he claims "cannot help us to do science."[17]

He also advocates “principles for an open future” which he claims underlie the work of both healthy scientific communities and democratic societies: “(1) When rational argument from public evidence suffices to decide a question, it must be considered to be so decided. (2) When rational argument from public evidence does not suffice to decide a question, the community must encourage a diverse range of viewpoints and hypotheses consistent with a good-faith attempt to develop convincing public evidence.” (Time Reborn p 265.)

Smolin was named as #21 on Foreign Policy Magazine's list of Top 100 Public Intellectuals.[18] In 2007 he was awarded the Majorana Prize from the Electronic Journal of Theoretical Physics, and in 2009 the Klopsteg Memorial Award from the American Association of Physics Teachers (AAPT) for “extraordinary accomplishments in communicating the excitement of physics to the general public,” He is a fellow of the Royal Society of Canada and the American Physical Society. In 2014 he was awarded the Buchalter Cosmology Prize for a work published in collaboration with Marina Cortês.[19]

Smolin was born in New York City. His father is Michael Smolin, an environmental and process engineer and his mother is the playwright Pauline Smolin. Lee Smolin has stayed involved with theatre becoming a scientific consultant for such plays as A Walk in the Woods by Lee Blessing, Background Interference by Drucilla Cornell and Infinity by Hannah Moscovitch.[20] He is married to Dina Graser, a lawyer and public servant in Toronto. His brother is law professor David M. Smolin.